Simulation of the TMI-2 Accident With ATHLET-CD and Analysis of the Late-Phase Modeling

This paper provides the results of a simulation of the TMI-2 accident with the current version of ATHLET-CD Mod. 2.2A as part of code validation activities at Ruhr-Universität Bochum (RUB). The calculated plant behavior during the first four phases of the accident is discussed and analyzed in comparison to available post-accident data and measurements. The calculation captures the plant response in terms of the thermal-hydraulics very well during the first two phases. However, during the reflooding of the degraded core some discrepancies between the calculation and TMI-2 data are identified. The code basically underestimates the hydrogen generation in this phase. Moreover, the debris bed and molten pool behavior during this phase cannot be simulated yet. An essential limitation of the current capabilities of the code in terms of the late-phase is the lack of models addressing the relocation of molten materials to the lower plenum of the reactor pressure vessel. Based on this analysis, the next steps needed to model the relocation of molten core components to the lower plenum are identified. These are the lateral leveling of accumulated molten material inside molten pools as well as the slumping to the lower plenum via different paths.